A team of astrobiologists pulled the bacteria from Mono Lake in eastern California and starved it of phosphate, the molecule most organisms prefer for building their DNA backbones, while force-feeding it arsenate, the analogous form of arsenic.

The bacteria continued to grow despite the poisonous diet, prompting the researchers to assert that the microbes had successfully swapped arsenic for phosphorous. The team, led by NASA astrobiologist Felisa Wolfe-Simon, published their results in Science Dec. 2, accompanied by a very excited NASA press conference.

But other biologists started raising red flags almost immediately, questioning the methods the team used to purify the DNA and asking why the researchers skipped certain tests.

"It seems much more likely that the arsenic they're seeing is contaminating arsenic that's going along for the ride," biologist Rosie Redfield of the University of British Columbia told Wired.com.

Redfield posted a biting critique Dec. 4 on her research blog. As of today, the post has received more than 40,000 hits.

She points out that the team didn't properly clean their DNA before or after running it through a standard device for separating DNA and RNA from other molecules, a technique called gel electrophoresis.

Cleaning the samples would require "a little kit that costs $2 and takes 10 minutes, and then you have pure DNA that you can analyze," Redfield said. The researchers used this method elsewhere in the paper, but not in the critical experiment that was supposed to show arsenic was incorporated into the bacteria's DNA.

"That's just asking for contamination problems," she said. The arsenic they found could have been hanging around in the gel, not in the cells, she added. "It's as if they wanted to find arsenic, so they didn't take a lot of trouble to make sure they didn't find it by mistake."

The NASA team immersed the DNA in water, where arsenic compounds quickly fall apart. If the DNA was really built from arsenate, it should have broken into pieces, Bradley wrote. But it didn't. That suggests the molecules were still using stronger phosphate to hold themselves together.

A similar story by Alla Katsnelson on Nature's news site points out the arsenate-eating microbes appear fat and bloated, a possible sign that they are sequestering toxic substances. Rather than continuing to grow in number and thrive, the bacteria could have been getting fatter as they stored up just enough energy for survival.

The authors of the original paper have so far declined to respond to these criticisms, at least to journalists. A NASA spokesperson also publicly dismissed blog-based critiques, saying any discussion should be confined to scientific journals.

"That's kind of sleazy given how they cooperated with all the media hype before the paper was published," Redfield said.

But senior author Ronald Oremland of the U.S. Geological Survey spoke to an audience of scientists at NASA Headquarters in Washington, DC Dec. 7, which was streamed live over NASA TV. Apparently against his own policy, Oremland fielded several questions about specific tests the team could have performed on the microbes.

Most of them, he said, were "certainly worth doing" and "an area for future work."

"There's a laundry list of things," he said. "We can't do everything."

Oremland also indirectly addressed the backlash in the blogs.

"I'm not surprised by pushback from the scientific community and bloggers. That's part of the process," he said. "But those are arguments about how many angels on the head of a pin. The only way this is going to get settled is if people reproduce these experiments on their own."

Redfield agrees that more studies are needed, and that the best place for scientific back-and-forth is in peer-reviewed journals.

"But putting it out just so people can comment on it directly is also extremely valuable," she said. Scientists have always discussed their work in non-peer-reviewed channels, such as letters or conferences, she notes. "It's just working faster and better now because we have things like blogs. That just lets the science be so much more powerful. In a lot of ways this is how science is supposed to work."

Image: The bloated microbes that may just be storing arsenic, not using it in their DNA. Science/AAAS